Various optical scanning devices used in laser beam scanning such as in copy machines, laser printers, and the like, or for recording or displaying picture images are conventionally known. Such optical scanning devices generally use a collimator lens to collimate the light beam received from, for example, a semiconductor laser, and are constructed so as to scan an image using a rotating multi-faceted mirror and an f.multidot..theta. lens. Collimator lenses consisting of two lens elements are disclosed in Japanese Laid Open Patent Applications S58-14109, S58-38915, S61-279820, S61-273520, and H2-73324. These collimator lenses are light in weight and compact.
When using a collimator lens in conjunction with an optical scanning device, the numerical aperture of the collimator lens that is employed is generally larger than that of the f 0 lens, in order to obtain high efficiency in gathering the light from a light source. However, such a design produces greater aberrations. Therefore, the aberrations of the collimator lens generally need to be corrected carefully.
Furthermore, in an optical scanning device, a multiple beam format is used for multi-beam scanning that uses multiple light sources, making it possible to either increase the scan speed or to make simultaneous recordings in one scan. In cases where a multi-beam format is used, favorable aberration correction is desirable within a half-picture angle so of two degrees.
With the above-mentioned collimator lenses, little consideration has apparently been given to the performance of the collimator lens at large picture angles. If the picture angle is large, since the various aberrations become troublesome, it becomes difficult to successfully use these lenses in a multibeam scanning optical system in which multiple light sources are arranged on a plane surface that is normal to the optical axis. In addition, in the case where use is made of a semiconductor laser as the light source, the phenomenon of mode hopping occurs as a result of changes in the ambient temperature or changes in the drive current. When mode hopping occurs, the output wavelength of the laser shifts, and this results in the position of the focus on the image-forming surface shifting due to the system magnification being dependent on the wavelength of the light. Thus, mode hopping makes it more difficult to record information with a high degree of resolution in that the various aberrations need to be well-corrected relative to the oscillation wavelength of the laser. However, the prior art collimator lenses referred to above do not adequately correct for aberrations due to a change in oscillation wavelength of the light source.